This invention relates to a process for treating a bearing surface, and particularly relates to such a process that causes partial melting of a bearing surface having microasperities so as to modify the topography of the surface for improved bearing performance.
One type of device for which a need exists to provide a carefully prepared bearing surface is a journal bearing drill bit used for drilling oil wells and the like. A very efficient type of such drill bit is depicted and described in U.S. Pat. No. 4,303,137, entitled "Method For Making A Cone For A Rock Bit And Product." Such a drill bit has a sturdy steel body which is threaded onto the lower end of a drill string and rotated in the hole being drilled. The drill bit has rolling cone cutters mounted on the body of the drill bit so as to rotate as the drill bit is rotated. Each of the rolling cone cutters rotates around a corresponding journal bering that in use is subjected to high radial and thrust loads. As the rolling cone cutters rotate in use, they crush and chip the rock at the bottom of the hole. Such rock drilling is very demanding service and the rock bit must have a rugged construction to withstand this demanding service. During manufacturing of such drill bits, substantial effort is made to prepare the bearing surfaces carefully; this effort is directed to minimizing the risk of premature bearing failure during service.
Each of the journal bearings comprises two relatively moving elements, a journal and a bushing, each of which is carefully machined to reduce friction between the relatively moving surfaces. Such careful machining cannot as a practical matter eliminate all contact between the relatively moving surfaces even when they are lubricated. Furthermore, the machining techniques such as grinding that are commonly used in manufacturing the bearings impart characteristics to the relatively moving surfaces that affect the dynamic friction coefficient of the bearing. One such characteristic is machine line orientation.
In a paper entitled "An Interferometric Study of the EHL of Rough Surfaces," by A. Jackson and A. Cameron, published in ASLE Trans., Vol. 19, No. 1, 1976, pp. 50-60, the authors report their study of the effects that transverse and longitudinal roughness have on EHL film shapes. Briefly, they found that under the same operating conditions, transverse one-dimensional roughness results in thicker oil films than longitudinal one-dimensional roughness.
The foregoing study was extended and conclusions were reported in a paper entitled "Effects of Asperities in Elastohydrodynamic Lubrication," by M. Kaneta and A. Cameron, published in Trans. ASME, Vol. 102, July 1980, pp. 374-379. Briefly, the conclusions were that both in rolling and sliding some asperity tips (i.e., micro protrusions on the journal or bushing surface) on the macro constriction are in a state of near contact when h/.sigma.&lt;2.5, where h is the central film thickness that would be obtained if the surfaces were smooth, and .sigma. is half the asperity height. A state of near contact on every asperity in the contact area is obtained at the condition of h/.sigma.&lt;1.2.
Because surface roughness and orientation of the machining lines relative to the sliding direction have the foregoing effects on lubricant thickness, they in turn affect bearing life. Furthermore, published results of experiments on friction behavior of surfaces containing artificially produced asperities and dents indicate that there is more to surface topography effect on bearing performance than just surface roughness. These publications include the above-cited paper by Kaneta and Cameron; a paper entitled "Elastohydrodynamic Film Thickness Measurements of Artificially Produced Nonsmooth Surfaces," by C. Cusano and L. D. Wedeven, published in ASLE Trans., Vol. 24, 1980, pp. 1-14; and a paper entitled "Microasperity Lubrication," by J. N. Anno, J. A. Walowit, and C. M. Allen, published in Trans. AMSE, J. of Lub. Tech., Vol. 90 (1968), pp. 1-14.
The above-cited paper by Anno et al. reports that surface microasperities in the form of circular cylinders that are 12.times.10.sup.-3 inches in diameter and 100 microinches in height have increased the load supporting capability of rotary shaft face-seals. It also reports that the load support is even further increased when the asperity tips are rounded. The above-cited paper by Cusano and Wedeven concerns work on surfaces containing artificially produced circular dents and indicates that h/.sigma. values as low as 0.69 did not produce any asperity-to-asperity contact even though, as mentioned above, at such low values of h/.sigma. full asperity contact is predicted. This is attributed to the increasing influence of microelastohydrodynamic (EHD) effects (due to micro dents) as the central film thickness, h is reduced.
Another relevant observation made in these papers is the fact that under sliding conditions surface asperities deform significantly as a result of micro-EHD pressure generation. As a consequence, for low values of h/.sigma., while a portion of the load supported by the lubricant may be increased by creation of micro-EHD conditions, i.e., fabrication of surfaces containing micro dents and asperities, the tendency to cause fatigue of asperities due to cyclic plastic deformation may be increased. In extreme cases micropitting and microcracking of the mating surfaces can occur. A desirable remedy for this problem is the creation of asperities that are rounded (i.e., no sharp angles to act as stress raisers), harder and under compressive stress. All three of these conditions are classical solutions to suspected metal fatigue problems.